1. Field of the Invention
The present invention relates to a method and a device for dynamic monitoring of gas sensors of an internal combustion engine, e.g., as exhaust gas sensors or as gas concentration sensors in a supply air channel, the gas sensors having a low-pass behavior, and in the event of a change of the gas state variable to be detected on the basis of a comparison of a modeled signal and a measured signal, a dynamic diagnosis being carried out.
2. Description of the Related Art
To reduce the emissions in passenger automobiles having gasoline engines, 3-way catalytic converters are generally used as emission control systems, which only convert a sufficient amount of exhaust gases if air-fuel ratio λ is regulated with high precision. For this purpose, air-fuel ratio λ is measured with the aid of an exhaust gas sensor situated upstream from the emission control system. The storage capacity of such an emission control system for oxygen is used for the purpose of absorbing oxygen in lean phases and discharging it again in rich phases. In this way, oxidizable harmful gas components of the exhaust gas may be may be converted. An exhaust gas sensor connected downstream from the emission control system is used for monitoring the oxygen storage capacity of the emission control system. The oxygen storage capacity must be monitored within the scope of the on-board diagnosis (OBD), since it represents a measure of the conversion ability of the emission control system. To determine the oxygen storage capacity, either the emission control system is initially supplied with oxygen in a lean phase and subsequently emptied in a rich phase having a lambda value known in the exhaust gas under consideration of the passing exhaust gas quantity, or the emission control system is initially emptied of oxygen in a rich phase and subsequently filled up in a lean phase having a lambda value known in the exhaust gas under consideration of the passing exhaust gas quantity. The lean phase is ended when the exhaust gas sensor connected downstream from the emission control system detects the oxygen which may no longer be stored by the emission control system. A rich phase is also ended when the exhaust gas sensor detects the passage of rich exhaust gas. The oxygen storage capacity of the emission control system corresponds to the quantity of reducing agent supplied during the rich phase for emptying or the quantity of oxygen supplied during the lean phase for filling. The precise quantities are calculated from the signal of the upstream exhaust gas sensor and the exhaust gas flow rate ascertained from other sensor signals.
If the dynamic of the upstream exhaust gas sensor decreases, for example, as a result of soiling or aging, the air-fuel ratio may no longer be regulated with the required precision, so that the conversion performance of the emission control system decreases. Furthermore, deviations in the diagnosis of the emission control system may result, which may have the result that an emission control system which is operating correctly per se is incorrectly evaluated as nonfunctional. Lawmakers require a diagnosis of the sensor properties during driving operation, to ensure that the required air-fuel ratio may still be set sufficiently precisely, the emissions do not exceed the permissible limiting values, and the emission control system is correctly monitored. The OBD-II standards require lambda sensors and other exhaust gas sensors to be monitored not only with respect to their electrical operational reliability, but rather also with respect to their response behavior, i.e., worsening of the sensor dynamic must be recognized, which may be made noticeable by an increased time constant and/or reaction time. Reaction times and delay times between a change of the exhaust gas composition and its recognition must be checked on-board as to whether they are still permissible for the user functions, i.e., for control, regulating, and monitoring functions, which use the sensor signal. The reaction time from a mixture change up to the signal edge and a specific rise time, for example, from 0% to 63% or from 30% to 60% of a signal deviation, are generally used as characteristic variables for the dynamic properties of exhaust gas sensors. The reaction time also includes the gas runtime from the engine outlet up to the sensor and accordingly changes in particular in the event of a manipulation of the sensor installation site.
In diesel engines, broadband lambda sensors and, in conjunction with SCR catalytic converters, also NOx sensors are used as gas sensors or gas concentration sensors. NOx sensors additionally also deliver an O2 signal. The O2 signal of the broadband lambda sensor or NOx sensor is used in the diesel engine not only for the operation of exhaust aftertreatment units, but rather also for the internal-engine emission reduction. The measured O2 concentration in the exhaust gas or the measured lambda signal is used to dynamically set the air-fuel mixture and thus minimize the scattering of the untreated emissions. In diesel engines having NOx storage catalytic converters (NSC), one broadband lambda sensor is required in each case upstream and downstream from the catalytic converter for a reliable description of the rich operation for regeneration. Internal-engine emission reduction and NSC operation also place specific minimum requirements on the dynamic properties of the O2 sensor. The rise time of the O2 signal is presently monitored during the transition from load to coasting, i.e., during the rise from a specific percentage below the normal O2 content of air to 21%. If the sensor signal does not once reach a specific intermediate value after a maximum time, this is interpreted as a reaction time error. In diesel engines having NOx storage catalytic converters (NSC), the response behavior of the lambda sensors upstream and downstream from the catalytic converter is generally also compared.
For upcoming vehicle generations or model years, it is to be expected that monitoring of the sensor dynamic upon falling O2 concentrations will also be required. In addition, in hybrid vehicles, there will no longer be coasting phases and therefore no phases having a constant O2 concentration of 21%. First approaches for these additional requirements are the active monitoring in published German patent application document DE 10 2008 001 121 A1 and the observer-based method in published German patent application document DE 10 2008 040 737 A1.
A method for monitoring dynamic properties of a broadband lambda sensor is known from published German patent application document DE 10 2008 040 737 A1, a measured lambda signal being determined with the aid of the broadband lambda sensor, which corresponds to an oxygen concentration in the exhaust gas of an internal combustion engine, an observer being assigned to the internal combustion engine, which generates a modeled lambda signal from input variables, and an estimated error signal being formed, as an input variable of a controller connected upstream in the observer from a model, from the difference of the modeled lambda signal and the measured lambda signal or from the difference of a signal derived from the modeled lambda signal and a signal derived from the measured lambda signal. It is provided that a measure of the dynamic properties of the broadband lambda sensor, which are characterized by a response time and a reaction time, is determined from an evaluation of the estimated error signal or a variable derived therefrom, and the measure for the dynamic properties is compared to predefined limiting values to evaluate to what extent the dynamic properties of the broadband lambda sensor are sufficient for a provided operation of the internal combustion engine.
In addition, a method and a device for online adaptation of an LSU dynamic model are described in published German patent application document DE 10 2008 001 569 A1. The publication specifically relates to a method and a device for adapting a dynamic model of an exhaust gas sensor, which is a component of an exhaust duct of an internal combustion engine and using which a lambda value for regulating an air-fuel composition is determined, a simulated lambda value being calculated in a control unit or in a diagnostic unit of the internal combustion engine in parallel thereto and both the simulated lambda value and the measured lambda value being used by an application function. It is provided that during ongoing vehicle operation, a jump behavior of the exhaust gas sensor is determined by analyzing a signal change upon excitation of the system and the dynamic model of the exhaust gas sensor is adapted on the basis of these results.
For the identification of the sensor properties, known functions for dynamic monitoring of broadband lambda sensors are resorted to. For other gas concentration signals of exhaust gas sensors, for example, for an NOx signal, comparable requirements apply as for O2 signals or O2 sensors. Similarities between the monitoring functions are therefore to be assumed.
The method as recited in published German patent application document DE 10 2008 001 121 A1 is an active monitoring. It includes an excitation by a test injection, which increases both the fuel consumption and the emissions. The method according to published German patent application document DE 10 2008 040 737 A1 does operate passively, but presumes a so-called observer, the application of which is complex. In addition, both methods are primarily targeted to the recognition of greater reaction time changes.